I too own both. The 5DIII has marginally better high-ISO performance, marginally higher resolution, slightly faster continuous shooting frame rate and a few nice little extras like dual card slots and in-camera HDR. (Of course, you can do HDR on the 5DII with exposure bracketing and a computer. The 5DIII saves the HDR only as a JPEG. For this reason, and for greater flexibility, I prefer to process my HDRs from the 5DIII on the computer anyway, using Photomatix.)

So if these are your criteria, it's probably not worth the extra to get the 5DIII.

However, where the 5DIII really shines is the improved auto focus, which is lightyears ahead of the 5DII. If you take photos where the AF isn't challenged, such as landscapes, and are happy to stick with using the most reliable focus point, the centre, then the 5DII will be fine. If you want more reliable and much more capable focusing then the 5DIII is definitely worth the extra.
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